In a balanced approach between technological advance and exploratory applications we plan: a) To develop new imaging methods based on ionic interaction, using the prototype low resolution scanning transmission ion microscope (STIM). A magnetic spectrometer will enable the separate detection of the dark field signals originating from proton-electron recombination (charge-exchange) and H2 ion dissociation, used here as image contrast mechanisms. Imaging by secondary electron emission induced by proton bombardment will also be developed. b) To improve the resolution of the STIM. This will first be attempted using the gun-microscope presently available by increasing source brightness and reducing beam aperture (limit estimated at approximately 200 A). Secondly, an einzel electrostatic lens will be introduced in the optical column (limit estimated at 20 - 50 A). c) To initiate exploratory biological investigations using the already established contrast mechanism due to critical range absorption. A systematic exploration of the ultrastructure of human prophase and metaphase chromosomes will be undertaken. We will correlate banding patterns observed by conventional techniques with STIM observations, and determine the extent of radiation damage due to proton bombardment, using SEM and TEM. The potential of proton microscopy as a tool in chromosome identification and as a source of new information will be assessed. We will also explore the feasibility and potential of cellular microradiography. Thin tissue sections of various tissues will be examined with the STIM, which will enhance the visibility of structures of different density, so that density abnormalities of pathological origin may become detectable.